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| 18 winding is set to zero. This means that there are only six possible current space vectors. Three Hall sensors are usually embedded in the rotor to provide positional information. When the rotor rotates, the current to the stator windings is switched every 60 degrees of rotation so that the current space vector is always within 30 degrees of the quadrature direction. Trapezoidal control can also be achieved without the Hall sensors by measuring the back-EMF generated by the rotation of the rotor to calculate the correct energizing sequence. As only two windings are energized at any point, no current flows in the non-energized winding, meaning that back-EMF can be directly sensed. Sensorless control is more complex, and the system needs to be tuned to the load profile of the application. However, it is less expensive to implement and requires less wiring than sensored control. This is important because some applications are not compatible with the extra wiring. Another option for controlling BDLC motors is using vector control, also known as field- orientated control (FOC). We will look at vector control in more detail in the AC Motors section, as it is most often used in those applications. Vector control performs complex calculations to allow the three-phase AC values to be handled like two- phase DC values. Because of its complexity, vector control requires very high-resolution positional information, a microcontroller with powerful processing ability, and a floating-point unit to perform the calculations. Vector control achieves high efficiency with a simpler physical design than sinusoidal control. It can also be implemented both with and without sensors for positional information. AC Motors AC motors also have a stator that creates a rotating magnetic field by energizing electromagnets in a defined sequence. The speed of the motor is tied to the frequency of the input, so if a change in rotor speed is required, the frequency of the current to the stator will need to be altered. There are two main ways of changing the frequency: scalar control and vector control. Scalar control is beneficial only for manipulating the magnitude and frequency of the stator voltage for applications with constant loads. Vector control provides independent control of the motor's speed and torque, making it possible to maintain a constant speed with varying loads. Both asynchronous and synchronous motors can use scalar and vector control. However, scalar control often leads to lower efficiency and a poor power factor. Vector control eliminates these problems but is more complex and expensive. Scalar Control Scalar control operates by keeping the magnetic field generated by the stator at a constant strength. To achieve this, the controller adjusts the voltage and frequency of the power delivered to the electromagnets on the stator. While changing the frequency alone alters the speed of the rotor, the voltage must be adjusted simultaneously to keep the torque stable. The most efficient way to change the speed without affecting the torque is by maintaining the V/ Hz ratio. For example, if the motor is rated for 120V/60Hz, it will always operate more effectively if a ratio of 2:1 is maintained. Scalar control is usually an open loop with an external stimulus, but closed- loop control is possible by adding encoder feedback to measure the actual speed of the rotor. However, closed-loop control is more expensive and complex because of the additional parts required. Vector Control Vector control allows the frequency and phase angle of the power delivered to the stator windings to be adjusted. It also controls the current's magnetic flux and torque components, making it the most complex control technique. As such, it is a very efficient and precise way to control the motor. It has become prevalent in recent years as legislation has been introduced to reduce the power consumed by motors. It also offers other advantages, including higher speeds and smoother torque. The torque in a motor is caused by the interaction of the electric fields generated by the rotor and stator and is at its highest when those fields are orthogonal. Vector control was developed to keep the two fields orthogonal during the motor's operation. Current in the stator windings couples the stator and rotor together, generating torque. Vector control separates these components and keeps the coupling current to a minimum while adjusting the